1. Field
The present invention relates to a mask frame assembly for thin film deposition.
2. Description of Related Art
Generally, organic light emitting display devices have superior characteristics such as wide viewing angles, high contrast ratios, and short response times.
Organic light-emitting display devices generally have a stacked structure including an anode, a cathode, and an emission layer located between the anode and the cathode. The devices display color images when holes and electrons, injected respectively from the anode and the cathode, recombine in the emission layer, which causes an emission of light. However, it may be difficult to achieve high light-emission efficiency with such a structure, and thus intermediate layers, including an electron injection layer, an electron transport layer, a hole transport layer, a hole injection layer, etc., are optionally located between the emission layer and each of the electrodes.
The electrodes and the interlayers may be formed using a variety of methods, such as a deposition method. When an organic light-emitting display device is manufactured using the deposition method, a fine metal mask (FMM) having the same pattern as a thin film to be formed contacts a substrate, and a thin film material is deposited over the FMM to form a thin film having the desired pattern.
As the size of the FMM increases, the possibility of an etching error occurring when the pattern is formed increases, and often, a middle portion of the FMM sags due to its weight. Thus, a divided mask formed by dividing a mask into several elongate or stripe-shaped portions and attaching them to a frame is often used. Although sagging of the divided mask occurs relatively infrequently compared to an undivided large-sized mask, sagging may still occur in the divided mask. Thus, to attach the divided mask to the frame, the divided mask is welded to the frame such that it is elastically extended in a lengthwise direction.
However, when the divided mask is welded to the frame while being extended in the manner described above, the frame may be deformed due to an elastic biasing force from the divided mask. When the frame is deformed, precision of mask patterns is degraded. Thus, in order to prevent deformation of the frame, a counter force is exerted on the frame when the divided mask is welded to the frame. In detail, the counter force is applied to the frame in a direction opposite to a direction in which the divided mask is elastically extended, so that when the extended divided mask is welded to the frame, even though the elastic restoring force of the divided mask is applied to the frame after welding has finished, the frame is not deformed. For example, when nine divided masks are attached to the frame, a counter force applied to the frame when a first divided mask is attached to the frame is largest, and the counter force to be applied gradually decreases as the number of divided masks is increased. Because the divided masks are welded to the frame while the counter force is continuously applied to the frame, as described above, deformation of the frame is suppressed even when the counter force is gradually decreased as the number of divided masks is increased. Thus, in an ideal case, after the last mask portion is welded to the frame, it may not be necessary to apply a counter force. Accordingly, after the last divided mask is welded to the frame, i.e., after a welding operation to weld the divided mask to the frame is completed, the frame is not be deformed because no biasing force or counter force is applied to the frame.
Because there is a small difference in tensile force due to a difference in characteristics of manufacturing each of the divided masks, even though the welding of the divided mask and the frame is performed in such a way that the counter force becomes zero in theory after the welding operation is completed, in reality, this is not the case. In other words, the counter force applied to the frame when each of the division frames is welded to the frame is calculated using a beam deflection theory, or the like. This means that the probability of accurately calculating the counter force when all of the divided masks have the same tensile characteristics is high, but in reality, characteristics of the divided masks are different from one another and thus the probability of accurately calculating the counter force is not perfect.
As such, the counter force may not become zero after the welding operation is completed and thus, the frame may be deformed. As a result, the precise patterns that should be formed by a deposition process may be distorted.